1. Field of the Invention
The invention relates to a voltage-controlled frequency oscillator comprising an amplifier device which has a main current path defined between a first and a second electrode, and a control electrode for controlling the conduction of this device, and comprising a capacitive divider bridge effecting a feedback coupling between the first electrode of the device and its control electrode, while one terminal of the divider bridge is connected to the control electrode and the other terminal is connected to a reference voltage and the intermediate junction point connected to the first electrode, said oscillator being also connected to a tuning circuit arranged between the control electrode and the reference voltage, which circuit comprises a first parallel branch constituted by a tuning inductance in series with a first variable capacitance and a second parallel branch equivalent to a second variable capacitance.
2. Description of the Prior Art
Such an oscillator of the Colpitts type is known from the document EP-A-0 270 298. It is particularly used as a local oscillator for receiving television signals.
A constant technical problem in this field is the tendency to cover a frequency range which is as extensive as possible when it is based on a given tuning inductance, while maintaining reliable oscillation conditions for the amplifier device. As the reactive impedance values are susceptible to variation in high proportions between the lowest frequency and the highest frequency in the envisaged frequency range, it is often necessary to use a tuning circuit having a relatively complex arrangement and realizing a satisfactory compromise between safe operation of the oscillator and the extent of the frequency range covered.
The known oscillator recommends the use of a tuning circuit having said second parallel branch so as to widen the frequency range covered by the capacitance variation of the variable capacitance diodes which constitute said variable capacitances. As far as the basic technical aspects of operation of such an oscillator are concerned, reference is made to the cited document EP-A-0 270 298. The difficulties of obtaining both an extensive range of frequencies and stability of operation of the oscillator throughout the extent of this range are explained in detail in this document whose contents are herein incorporated by reference. It has been stated that the known oscillator is capable of covering various frequency ranges, the most extensive having 263 and 509 MHz as their extreme values, i.e. a ratio of 1.93 between these frequencies. It is desirable to extend these possibilities and to achieve a ratio of at least 2.7 between the extreme frequencies in the same range.